Make line_of_sight more useful, return blocking node position when false

A simple modification to return potentially useful information which is currently discarded.

Forum topic: https://forum.minetest.net/viewtopic.php?id=7857

Example code:

	local success, pos = minetest.get_line_of_sight(pos1, pos2)
	if success == false then
		print(dump(pos))
	end

Possible uses might include shooting, long range pointing, sonar, sight though glass etc.
These are just some ideas, I am sure there will more.